Cyclotrons are circular accelerators allowing the acceleration of charged particles such as positive ions (protons, deuterons, helions, alpha particles, etc.) or negative ions (H″, D−, etc.), which are utilized among others for producing radioactive isotopes, for radiation therapy, or for experimental purposes.
The first cyclotrons comprised a magnetic circuit that was simply comprised of two symmetrical poles disposed on both sides of a median plane and separated by a gap in which the accelerated particles circulate. The magnetic circuit is supplemented by flux returns in order to close said circuit and cylinder heads used as base plates at the poles. The poles are surrounded by a pair of induction coils supplied by a current, which generates a uniform and constant magnetic field that is capable of confining the particles according to an essentially circular trajectory or more precisely according to a trajectory in the form of a spiral in the median plane.
In an improved variation, azimuthal field variation machines are known. The poles of the electromagnet are then divided into sectors alternatively presenting a smaller gap and a larger gap. The azimuthal field variation that results has the effect of ensuring the vertical and horizontal focalization of the beam during acceleration.
Among the azimuthal field variation cyclotrons, one must distinguish between compact type cyclotrons, whose field is created by a pair of circular main coils, from cyclotrons with separate sectors, in which the magnetic structure is divided into entirely autonomous separate units, where each pair of poles disposes its own coils.
Document EP-A-0222786 describes an example of a compact isochronous cyclotron.
A large field of application for cyclotrons is the utilization of accelerated particles to bombard targets in order to produce radioisotopes. In this object, one may extract said accelerated particle beam from the cyclotron. Among the extraction methods, a known method is the method of extraction by “stripping.” The accelerated particles are most often negatively charged ions comprised of a nucleus and several electrons.
In the vicinity of the periphery of the cyclotron, the beam is directed towards a thin sheet, called the “stripping sheet,” generally made of carbon. This stripping sheet has the effect of stripping the peripheral electrons from the ions, thus changing their charge. The trajectory curve is thus inverted and the beam is directed to the outside of the machine, by an opening made in the flux return of the magnetic circuit.
Another known method of extraction of the beam is auto-extraction, by means of an abrupt radial variation in the induction field at the periphery of the cyclotron. This method is described in detail in documents WO A-97/14279 and WO-A-01/05199.
For the particular application of producing radioisotopes, the beam of charged particles is directed towards a target containing at least one precursor element of the radioisotope to be produced. In this case, it is particularly desirable that the beam be directed towards the center of the target.
An element limiting the productivity of the radioisotope production system is the capacity of the target to dissipate the thermal capacity that the target receives by the beam. If said target receives an intensity that is too strong from the beam (or current), it risks being damaged. For some types of targets, irradiation intensities are limited to 40 μA, while cyclotrons used in nuclear medicine are capable of producing beams with intensities that may reach 80 to 100 μA. Therefore, one may not fully utilize the production capacities of the cyclotron in this scenario, essentially due to the fact that one cannot manage to sufficiently cool the target.
In the object of increasing the productivity of a system for producing radioisotopes while not exceeding the limit of acceptable current for a target, double beam systems have been proposed. According to such a configuration, two stripping sheets are disposed at the periphery of the cyclotron in a diametrically opposed manner with relation to the central axis of the machine. The beam is thus divided into two roughly equal fractions. Nevertheless, owing to, for example, a defect in the symmetry of the cyclotron, it could be that one of the targets receives a beam intensity that is essentially different from that received by the other target. Then it may happen that one of the targets could be damaged by a current that is too strong. This situation could be produced in particular when, in the course of a lengthy irradiation, for example of several hours, some machine settings thus undergo a drift, particularly following a progressive heating of its elements.
To resolve this problem, proposing stripping sheets that are radially displaceable is known. This solution is used for example in the Cyclone 30 machine of the Applicant. By a radial displacement of the stripping sheet towards the inside or outside of the cyclotron, one increases or reduces the fraction of the beam intercepted by the sheet. In a double beam machine, one may, by displacing one of the two sheets towards the inside and the other sheet towards the outside, ensure the balanced distribution of the intensity of the beam hitting each of the targets. However, this solution is delicate and costly, since it requires the installation of adjustable mobile equipment within the same machine, that is, in the vacuum chamber.
The utilization of harmonic coils has also been proposed in order to make the two beams of particles issued from the same double beam system essentially equivalent, that is, presenting an equivalent intensity. According to this solution, disposing small-size harmonic coils between the poles of the electromagnet has been proposed. Opposite currents flow through two coils, which produces an increase in the magnetic field in a region of the gap, and a reduction in the magnetic field in the region of the diametrically opposed gap. This solution thus allows the intensity of the beams to be adjusted, but presents the following disadvantages: in particular, the harmonic coils must be located at the hills, where the gap is the narrowest. Thus, the coils may be directly reached by the beam, more particularly in the case of a defect in the axial alignment of said beam, which will inevitably lead to the destruction of said coils. Furthermore, as these coils are disposed in the vacuum chamber, the conductors powering these coils must traverse the wall of said chamber by means that respect a complete leakproofness, which may pose difficulties.
A third solution that is known and already utilized by the applicant is illustrated in FIG. 1. If one causes the high-frequency alternating current voltage applied to the acceleration electrodes (the dees) to be varied, one observes the following situation: if the amplitude of the high-frequency voltage applied to the dees (Vdee) is progressively increased, a corresponding increase in the total intensity of the beam produced by the cyclotron is observed, which is explained by the increase in effectiveness of the ion supply with this voltage. One also observes, as is shown in FIG. 1, that the intensities reaching each of the targets fluctuate around an average value, and that for some specified Vdee values, where the curves intersect, the intensities are equal. Thus it suffices to choose the Vdee voltage that is equal to one of these values to equalize the intensity of the beam reaching each of the targets. However, cases where these two curves never intersect have been observed following thermal drift, or due to dissymmetries in the construction of the cyclotron. It is then impossible to equalize the currents hitting the two targets by this method.